Give better caching directives for static files in web interface

Description

It looks like my web browser is requesting /default.css and /bg_gradient.jpg every single time I switch to a new page. Even if I use back and forward buttons, which don't cause the page to be requested again, it still requests again those two files.

The requests properly use If-Modified-Since, and buildbot properly replies with 304 Not Modified, but it's still annoying. Clutters the webserver log, for one.

According to ​redbot, the responses are “stale” as soon as they go out. They should have an appropriate Expires header, or Cache-Control: max-age=3600 (1 hour expiration from now).